DNS in Windows XP

When you visit a website, Windows XP tries to speed things up by placing the DNS information about that site, into a DNS cache on your PC.

This is OK until there are problems with the network, DNS server or even the website!

You see the DNS Windows XP cache stores BOTH negative and positive entries. The positive entries are those that the DNS lookup was successful and you could connect to the website.

Negative entries are those where the DNS lookup failed and you could not connect to the website.

A problem can arise here because if the DNS cache hold a negative entry and the website you want to view is now OK to view, Windows XP will still give you a DNS error!

By default these negative entries are cached for 5 mins. But we can tweak the registry to NOT store negative entries at all!

There is ALSO another potential pitfall with the Windows XP DNS cache...

Most ISP's and Domain name registrar's now update their DNS information (TTL) every 4 hours instead of the twice daily updates that were common few years ago.

This means that your DNS Windows XP cache may be out of date for certain entries!

So lets look at how we can tweak the registry to prevent these problems occurring:

Flush DNS

First of all it would be a good idea to flush the local DNS cache so we are starting with a clean sheet!

To do this simply enter: ipconfig /flushdns in a command prompt. You will then get the following window appear confirming the flush,

Successful DNS Flush

Now we have a clean DNS cache lets make the changes to the registry.

Block Negative Entries

To force Windows XP NOT to cache negative entries we need to add a new DWORD to the following Registry key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet
\Services\Dnscache\Parameters

DWORD: MaxNegativeCacheTtl

Value: 0

This will now ensure NO negative entries are stored.

Cache TTL

To force Windows XP to keep positive entries in DNS Cache for only 4 hours instead of the default 24 hours we need to apply the following change to the registry:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\Dnscache\Parameters

DWORD: MaxCacheTtl

Value: 14400

NB - make sure you enter the 14400 (seconds) as Decimal Base and NOT Hexadecimal Base!

Now your registry key should look similar to this:

Thanks for reading and I hope this article on the DNS Windows XP cache was useful!

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